Rotary valves can provide a convenient and compact way of consolidating the multiple valves required for repetitive chemical processing cycles into a single, simple unit. For instance, rotary valves can be particularly useful in chemical processing operations involving pressure swing devices (e.g. pressure swing adsorption devices, pressure swing reforming devices).
In general, rotary valves comprise a stator and a rotor that is rotated about its axis relative to the stator. Both stator and rotor contain suitably sized and located ports that function as multiple valves as a result of the rotation of the rotor. Via this rotation, the ports in the rotor come into and out of alignment with ports in the stator, thus opening and closing the ports to fluid flow, and thereby serving as valves. Some means of loading is required to engage the rotor with the stator in order to maintain an adequate seal therebetween. In some applications, a uniform, constant load may provide satisfactory function of the rotary valve.
However, in pressure swing applications, the fluid pressure present at one port in the valve may be vastly different from the pressure at another location in the valve (e.g. the pressure may vary by tens of atmospheres). Such an imbalance cannot be readily accommodated with the use of a uniform, constant sealing load. And, in order to maintain a satisfactory seal in a region of high fluid pressure, a correspondingly large sealing load must be applied there to counteract the pressure forcing the stator and rotor apart. However, in regions of relatively low fluid pressure, the same large sealing load would result in great friction between stator and rotor and hence increased heating, wear, and torque requirement for the motor driving the rotor. In these pressure swing applications then, it is thus preferred to employ pressure balanced rotary valves in which variable loading pressures are employed at the various port locations such that higher sealing or closing loads are provided in regions of high pressure and lower sealing/closing loads are provided in regions of low pressure.
A particularly suitable pressure balanced rotary valve for use in pressure swing devices is disclosed in U.S. Reissue Pat. No. 38493. Therein, the rotary valve comprises axially aligned fluid transfer sleeves (e.g. pistons) which are used to provide a variable load to keep the stator and rotor closed. Mechanical springs may be employed to provide a minimum fixed load to keep the stator and rotor closed. In particular, the fluid transfer sleeves may be located in cylinders formed in the stator at the various locations of the stator ports. The sleeves may be sealed to the cylinders using static seals or piston rings. Each adsorbent bed in the pressure swing device is connected to one of these ports and thus the fluid pressure in the fluid transfer sleeve is provided by that adsorbent bed fluidly connected to it. A variable load is thus provided at each port in the stator and this load is a function of the pressure in the adsorbent bed and of the axial area of the fluid transfer sleeve. In U.S. Pat. No. 38,493, mechanical springs provide a fixed load to the stator via the fluid transfer sleeves (e.g. springs are located beneath the fluid transfer sleeves and assist the sleeves to urge the stator against the rotor).
Commercial pressure swing adsorption devices employing such pressure balanced rotary valves have been available for many years now (e.g. H3200 series PSA devices of Xebec Adsorption Inc., formerly Questair Technologies Inc.). In these commercial devices, the mechanical springs in the fluid transfer sleeves are generally selected to apply a sufficient fixed load to obtain a satisfactory seal on startup of the device. Otherwise though, the fixed load is generally set to be low as possible to keep friction heating, wear, and torque to a minimum.
In rotary valves of this kind, the ports in both stator and rotor may be located on a port pitch circle that is centered on the rotor axis. In typical commercial applications, the operating parameters required of the device (e.g. flow rates, pressures, and the like), combined with the preference for keeping the fixed load as low as possible, generally result in a relatively large fluid transfer sleeve or piston size being employed. Because these pistons are also located on, or close to, the ports, the relatively large piston size typically limits the minimum port pitch circle that can be used. As a consequence, the relatively smaller stator ports wind up spaced apart such that the spacings between stator ports are greater than the size of the ports themselves.
In addition, the typical rotary valve pressure swing adsorption device employs two rotary valves, one connected to the feed ends of the adsorbent beds where the gas mixtures to be separated are provided, and another connected to the product ends of the adsorbent beds where separated product gases are obtained. The port geometries in the stators and rotors (i.e. size, shape, and spacing) are typically chosen to optimize the process cycle for a given gas separation application. However, this too would generally include choosing a design in which the port spacings on the stator are greater in size than the ports themselves. On the product end of the device, this has been an essential requirement. This is because the ports in the rotor (which are typically as big or bigger than those in the stator) would otherwise interconnect adjacent stator ports over portions of the rotation cycle and thus cause an unacceptable bridging of gases between these adjacent stator ports.
In a given chemical processing application, if greater processing throughput is desired, the rotary valves must be designed to handle greater gas flows and hence employ larger ports. Conventionally, this means the rotary valve diameter would be increased accordingly to accommodate the greater port size. Also, the size of the fluid transfer sleeves or pistons must be increased accordingly to provide a greater possible variable load for pressure balancing purposes.